The invention relates to a method to verify that a service transmitted on an alternative frequency is the same as a service transmitted on said currently received frequency. In particular to such a verification for the purpose to perform a seamless switching from a first currently tuned frequency to a second alternative frequency (AF).
In broadcast systems that deliver the same services in adjacent or overlapping areas on different frequencies, it is needed to find a proper criteria to switch to an alternative frequency without loosing the service, i. e. to perform a seamless switching.
In public information service systems like DAB or DVB-T techniques for switching to alternative frequency are used, but they provide no disturbance-free switching from one frequency to another. In the EP-A-98 119 400 a method and data frame structure for the digital transmission of information is suggested in which the transmission system is defined such that the receiver is able to test an alternative frequency without loosing any relevant information on the current tuned frequency, because the signal in the air consists of two parts, namely a continuous data-channel like audio with interleaving in time, but not repeated, and a static data channel including information about the service, multiplex configuration, program time, transmitter ID, service ID and alternative frequency list. In this system the receiver has the time to check alternative frequencies without loosing relevant information data during the static data-channel.
However, this transmission system underlies the condition that the static data-channel is identical and unique for all services at all times, i. e. the same static data-channel is transmitted by all transmitters belonging to a service without any changes at any time. For a certain radio transmission systems, e. g. DRM (Digital Radio Mondial), no such reliable static data-channel is provided and therefore it cannot be secured that in such radio transmission systems a seamless switching will be performed in any instance.
Therefore, the EP-A-99 109 102 provides a method to perform a seamless switching from a first currently tuned frequency to a second alternative frequency with the step of receiving at least one set of samples from a respective signal transmitted on at least one second frequency during a time period during which an indicator assures that it is secure that only data that has been transmitted at least once is transmitted as signal on said first frequency. Such a radio transmission signal consists of a quasi-static data-channel (SD), a dynamic data-channel (DD) and a gap-channel (GAP). The signal is then formed of consecutive frames each of which consists of a gap part, a quasi-static data part and a dynamic data part. In this case, a respective indicator within a respective dynamic data part about the quasi-static data part relates also to a forthcoming gap part transmitted in the same signal frame as the symbol(s) of the quasi-static data part the respective indicator relates to.
The checking itself whether the signal transmitted on the alternative frequency is the same as the signal on the currently received frequency, wherein one of the signals can have a time delay, is performed with the help of a correlation of a reference signal generated on basis of the signal received on the currently tuned frequency with a signal probe, i. e. a set of samples, of the signal received on the alternative frequency. Especially, when the set of samples consists only a small number of samples there might be the probability that the AF transmits an other service even if a correlation peak occurs.
Therefore, it is the object of the present invention to provide a verification method that a signal transmitted on the alternative frequency is the same as the signal transmitted on said currently received frequency, i.e. that both frequencies transmit the same service.
This object is solved by a method according to independent claim 1, preferred embodiments thereof are defined in the respective subclaims.
The present invention provides a method to verify that a service transmitted on an alternative frequency to a currently received frequency is the same as a service transmitted on the currently received frequency, wherein a first correlation result of a first set of samples taken from a signal representing the service transmitted in the alternative frequency at a first point of time with a reference signal indicates whether or not both signals are the same The method comprises the steps of calculating a respective further correlation result of at least one father set of samples taken from the signal transmitted on the alternative frequency at a further point of time that has a respective predetermined time relationship (xcex94tcorr) to the first point of time with the reference signal, and comparing the first correlation result with the respective further correlation result, wherein the comparing result verifies whether or not both services are the same.